So for the new God-Eternal Kefnet, I have a simple question. For his ability that triggers on the first card drawn each turn, do you get to see the card before you reveal it, and then choose to reveal it or not after looking at it first? Or, do you choose whether you're going to reveal the card before you actually draw it, and then either do or don't based on which you chose?
A game mechanic of the form "You may reveal [a card] as you draw it" on a static ability, such as Kefnet's second ability, is novel. It remains to be seen whether the War of the Spark release notes or the comprehensive rules will clarify your question. But see comment 3.
However, under current rules, in the specific case that "a spell or ability causes a card to be drawn while another spell is being cast" or another ability is being activated (e.g., if you tap Chromatic Sphere for mana while casting Cancel)—
that card "is kept face down until that [other] spell becomes cast" or that other ability becomes activated, and
if "an effect allows ... a player to reveal the card as it's being drawn, it's revealed after the spell becomes cast or the ability becomes activated"
(C.R. 120.8), so that in this specific case, you won't know what that card is by the time you draw it this way, even if you control Kefnet and it's the first card you've drawn this turn.
A similar mechanic to that found in Kefnet's second ability occurs in several other cards, notably Counterbalance and the ripple ability (C.R. 702.59a), and at least there, the choice of whether to reveal the card or cards in question is made "while applying the effect" (C.R. 608.2d). See also C.R. 101.4, which covers choices to be made at the same time by multiple players. Thus, based on those other cards, you don't necessarily get to know the identity of the top card of your library by the time you choose whether to reveal that card due to Kefnet's second ability.
EDIT: Edited after comment 3 was posted.
EDIT (May 3): The updated rules for War of the Spark are now available. C.R. 120.9 is a new rule that backs comment 3. Struck out some text that's inconsistent with that new rule (", so that in this ... drawn this turn").
It remains to be seen whether the War of the Spark release notes or the comprehensive rules will clarify your question.But see comment 3.However, under current rules, in the specific case that "a spell or ability causes a card to be drawn while another spell is being cast" or another ability is being activated (e.g., if you tap Chromatic Sphere for mana while casting Cancel)—
, so that in this specific case, you won't know what that card is by the time you draw it this way, even if you control Kefnet and it's the first card you've drawn this turn.A similar mechanic to that found in Kefnet's second ability occurs in several other cards, notably Counterbalance and the ripple ability (C.R. 702.59a), and at least there, the choice of whether to reveal the card or cards in question is made "while applying the effect" (C.R. 608.2d). See also C.R. 101.4, which covers choices to be made at the same time by multiple players. Thus, based on those other cards, you don't necessarily get to know the identity of the top card of your library by the time you choose whether to reveal that card due to Kefnet's second ability.EDIT: Edited after comment 3 was posted.
EDIT (May 3): The updated rules for War of the Spark are now available. C.R. 120.9 is a new rule that backs comment 3. Struck out some text that's inconsistent with that new rule (", so that in this ... drawn this turn").
https://twitter.com/EliShffrn/status/1116577296343678978